1. Field of the Invention
The present invention relates generally to the field of downhole sampling and in particular to the continuous measurement of parameters of interest and on site analysis for hydrocarbon samples after capture in a downhole sample chamber to insure the integrity of the sample until transfer to a laboratory for analysis of the sample.
2. Summary of the Related Art
Earth formation fluids extant in a hydrocarbon producing well typically comprise a mixture of oil, gas, and water. The pressure, temperature and volume of formation fluids in a confined space determine the phase relation of these constituents. In a subsurface formation, high well fluid pressures often entrain gas within the oil above the bubble point pressure. When the pressure is reduced, the entrained or dissolved gaseous compounds separate from the liquid phase sample. The accurate measure of pressure, temperature, and formation fluid composition from a particular well affects the commercial interest in producing fluids available from the well. The data also provides information regarding procedures for maximizing the completion and production of the respective hydrocarbon reservoir.
Certain techniques facilitate analysis of the formation fluids downhole in the well bore. U.S. Pat. No. 6,467,544 to Brown, et al. describes a sample chamber having a slidably disposed piston to define a sample cavity on one side of the piston and a buffer cavity on the other side of the piston. U.S. Pat. No. 5,361,839 to Griffith et al. (1993) disclosed a transducer for generating an output representative of fluid sample characteristics downhole in a wellbore. U.S. Pat. No. 5,329,811 to Schultz et al. (I 994) disclosed an apparatus and method for assessing pressure and volume data for a downhole well fluid sample.
Other techniques capture a well fluid sample for retrieval to the surface. U.S. Pat. No. 4,583,595 to Czenichow et al. (1986) disclosed a piston actuated mechanism for capturing a well fluid sample. U.S. Pat. No. 4,721,157 to Berzin (1988) disclosed a shifting valve sleeve for capturing a well fluid sample in a chamber. U.S. Pat. No. 4,766,955 to Petermann (1988) disclosed a piston engaged with a control valve for capturing a well fluid sample, and U.S. Pat. No. 4,903,765 to Zunkel (1990) disclosed a time delayed well fluid sampler. U.S. Pat. No. 5,009,100 to Gruber et al. (1991) disclosed a wireline sampler for collecting a well fluid sample from a selected wellbore depth, U.S. Pat. No. 5,240,072 to Schultz et al. (1993) disclosed a multiple sample annulus pressure responsive sampler for permitting well fluid sample collection at different time and depth intervals, and U.S. Pat. No. 5,322,120 to Be et al. (1994) disclosed an electrically actuated hydraulic system for collecting well fluid samples deep in a wellbore.
Temperatures downhole in a deep wellbore often exceed 300 degrees F. When a hot formation fluid sample is retrieved to the surface at 70 degrees F., the resulting drop in temperature causes the formation fluid sample to contract. If the volume of the sample is unchanged, such contraction substantially reduces the sample pressure. A pressure drop changes in the situ formation fluid parameters, and can permit phase separation between liquids and gases entrained within the formation fluid sample. Phase separation significantly changes the formation fluid characteristics, and reduces the ability to accurately evaluate the actual properties of the formation fluid.
To overcome this limitation, various techniques have been developed to maintain pressure of the formation fluid sample. U.S. Pat. No. 5,337,822 to Massie et al. (1994) pressurized a formation fluid sample with a hydraulically driven piston powered by a high-pressure gas. Similarly, U.S. Pat. No. 5,662,166 to Shammai (1997) disclosed a pressurized gas to charge the formation fluid sample. U.S. Pat. No. 5,303,775 (1994) and U.S. Pat. No. 5,377,755 (1995) to Michaels et al. disclose a bi-directional, positive displacement pump for increasing the formation fluid sample pressure above the bubble point so that subsequent cooling did not reduce the fluid pressure below the bubble point.
Due to the uncertainty of the restoration process, any pressure-volume-temperature (PVT) lab analyses that are performed on the restored sing-phase crude oil are suspect. When using ordinary sample tanks, one tries to minimize this problem of cooling and separating into two-phase by pressurizing the sample down hole to a pressure that is far (4500 or more psi) above the downhole formation pressure. The extra pressurization is an attempt to squeeze enough extra crude oil into the fixed volume of the tank that upon cooling to surface temperatures the crude oil is still under enough pressure to maintain a single-phase state and maintains at least at the pressure that it had downhole.
The gas cushion of the single-phase tanks, thus, makes it easier to maintain a sample in a single phase state because, as the crude oil sample shrinks, the gas cushion expands to keep pressure on the crude. However, if the crude oil shrinks too much, the gas cushion (which expands by as much as the crude shrinks) may expand to the point that the pressure applied by the gas cushion to the crude falls below formation pressure and allows asphaltenes in the crude oil to precipitate out or gas bubbles to form. Thus, there is a need to monitor the integrity of the sample from the time the sample is brought to the surface until it is delivered to the laboratory for analysis.